"The methodology, the construction of this appalling camp in Cuba, is tantamount to a camp that is there for the sole purpose of torture. There is no other," she said.

Mr Begg is one of four Britons being held at Guantanamo Bay.

Also detained are Feroz Abbasi, Martin Mubanga and Richard Belmar, who are all from London.

Mr Begg's letter, dated 12 July 2004, was an "oddity" that may have reached them unclassified and uncensored by the US by mistake, said his lawyers.

'Duress'

In it he demanded his rights under US and international law, saying he should be released immediately.

All charges against him should be presented "unambiguously," he added.

Mr Begg asked in the letter for answers over the "violations and abuses" he has suffered.

Any statements he had signed were done so "under duress", he added, insisting he was an innocent "law-abiding citizen of the UK".

During interviews - mostly, but not solely, in Afghanistan - he claimed he was subjected to threats of, and actual, torture.

He had also faced religious and racial abuse, he said.

Welfare visits

He claimed he had heard other detainees being tortured, and believed this had led to the deaths of two men, which he said he "partially witnessed".

Ms Peirce said the letter would explain why Mr Begg was the only detainee who had not been seen by others.

"I would say that he has been held separately because he has witnessed murder, and therefore he is in a unique position."

Copies of the letter have been sent to Tony Blair, Home Secretary David Blunkett, Foreign Secretary Jack Straw and the Attorney-General Lord Goldsmith, said Mr Begg's legal team.

The Foreign Office said the welfare of British detainees at Guantanamo had been a priority for the government "from the outset" and that it continued to pursue complaints raised with the US authorities.

It said during welfare visits to the military base, Mr Begg had not alleged systematic abuse there, but had claimed mistreatment at Bagram, which was being investigated by the US.

Mr Begg was arrested in Pakistan before being moved to Cuba.

Five other Britons who spent up to two years at Guantanamo Bay were handed over to British custody in March and then freed without charge.